A vinyl chloride polymer has been widely used in various industrial fields including the production of pipe and construction materials owing to its excellent mechanical strength, weatherability and chemical resistance. However, its moldability is not very good and needs to be improved. In general, the moldability of a vinyl chloride polymer depends on the polymer particle characteristics. Therefore, in order to increase productivity, the bulk specific gravity, internal porosity, diameter and particle distribution of vinyl chloride polymer particles need to be controlled, along with the improvement of gelation characteristics, plasticizer absorptiveness and powder fluidity. In particular, the bulk specific gravity of the vinyl chloride polymer must increase to improve the productivity from extrusion molding, and the melting rate of gelation of the vinyl chloride polymer must be accelerated to improve the processability of extrusion molding. However, when extrudate produced from extrusion molding is increased, the melting torque is increased. But, melting torque needs to be low for gelation of the vinyl chloride polymer. The conventional suspension polymerization produces vinyl chloride having either excellent bulk specific gravity or excellent gelation properties, which is not satisfactory for extrusion molding.
As an alternative to improve the processability of a vinyl chloride polymer molding, it has been proposed that 0.01˜0.2 weight part of one or more partially saponificated polyvinyl alcohols having 65 mol % hydration and an average degree of polymerization of 500˜4000 is added to 100 weight part of vinyl chloride monomer as a dispersion stabilizer for polymerization, followed by suspension polymerization, and then the reaction mixture is cooled down in a reflux condenser when the conversion rate of the vinyl chloride monomer reaches 3˜40%, during which a polyvinyl alcohol having at least 97 mol % hydration and an average degree of polymerization of at least 1500 is additionally added (JP 1998-045813). However, the resultant vinyl chloride polymer of this proposed method has the problem of decreased melting properties because of the addition of a dispersion stabilizer having 90 mol % hydration, even though the bulk specific gravity of the vinyl chloride polymer can be increased by this method.